1. Field of the Invention
This invention relates to a method of forming two-dimensional gradated images by using a thermal printer, and to thermal head for thermal printers.
2. Description of the Prior Art
There are a dry silver process and a discharge breaking system which are known as methods of recording a gradated video signal as a two-dimensional image but these methods have both merits and demerits.
In the dry silver systems, an image of non-stepped gradation can be obtained but the image is readily affected by an ambient temperature. Consequently, it is difficult to record an image of a uniform quality.
In the discharge breaking system, an image of a comparatively uniform quality is easily obtained but an image of over eight-stepped gradation cannot be obtained. It is difficult to improve the resolving power in this system.
A known thermal printer used to record letters and figures permits reproducing an image of a substantially non-stepped gradation and does not require a developing step. Consequently, there is the possibility that a two-dimensional gradated image is recorded by such a printer. The thermal printer referred to above includes a thermal head C as shown in FIG. 2, which consists of a ceramic substrate A and a plurality of heating elements lined up on the surface of the substrate A. This thermal head C traverses the surface of thermal paper D in a primary scanning direction X as shown in FIG. 1. Synchronously with the scanning movement of the thermal head C, the thermal paper D is fed intermittently in a secondary scanning direction Y by a distance slightly greater than length L.sub.1 of arrangement of the heating elements B.
The inventors have discussed the recording of a two-dimensional image of a high gradation by a thermal printer of the above-mentioned construction, to find out that what does not cause any trouble in the recording of letters and figures hampers the recording of a two-dimensional image of a high gradation.
When a two dimensional gradated image is formed by inputting a video signal into a thermal printer mentioned above, transversely extended, parallel, white lines are drawn on such portions of the surface of thermal paper D that are opposed to spaces P among heating elements B, and also on such portions of the thermal paper D that correspond to positions where each of secondary scanning movements of thermal head terminates. These white lines cause no trouble in the recording of letters and figures because letters and figures have no gradation. In a gradated image, however, such white lines cause a remarkable contrast between image elements of different densities. Such an image is hard to observe. This is fatal to the formation of a high-quality image. Especially, when thermal paper D is fed irregularly, white lines of inconstant width are formed. A recorded image having such white lines is commercially valueless.